docs: def annotate_distance_in_km(self, latitude, longitude):
          """
          Return all records with non-null latitude/longitude values with the
          annotation value `distance_in_km` which is the distance between
          the record and the given `latitude`/`longitude` location.
          """
          # Great circle distance formula, taken on faith from StackOverflow link
          # above, see also https://en.wikipedia.org/wiki/Great-circle_distance
          # NOTE: We use psql-specific COALESCE() to choose marker lat/long
          # values when available (non-NULL) otherwise center lat/long values
          GCD = """
              6371
              * ACOS(
                  COS(RADIANS(%s))
                  * COS(RADIANS(
                      COALESCE(map_marker_lat, map_center_lat)
                    ))
                  * COS(RADIANS(
                      COALESCE(map_marker_long, map_center_long)
                    )
                  - RADIANS(%s))
                  + SIN(RADIANS(%s))
                  * SIN(RADIANS(
                      COALESCE(map_marker_lat, map_center_lat)
                  ))
              )
              """
          return self.get_queryset() \
              .exclude(map_center_lat=None) \
              .exclude(map_center_long=None) \
              .annotate(
                  distance_in_km=models.expressions.RawSQL(
                      GCD,
                      (latitude, longitude, latitude)
                  )
              ) \
              .order_by('distance_in_km')